The Quincy Massacre was a battle between the Commonwealth Minutemen and the Gunners in the town of Quincy, a settlement allied with the Commonwealth Minutemen. The Gunners' attack was very successful, resulting in the destruction of the Commonwealth Minutemen with only twenty surviving (including Preston Garvey), and the occupation of Quincy as a permanent Gunner stronghold.
In 2287, in response to a vision of Mama Murphy's, Sturges the mechanic requested aid from the Commonwealth Minutemen. A small detachment of about a dozen Minutemen under the command of Colonel Ezra Hollis, some of the most notable in their number being Preston Garvey, Anthony, Josh, Emma, and Clint (at least initially) arrived to defend the town. This turned out to be in the nick of time, as a large force of Gunners led by Baker were already laying siege to it. The Minutemen were initially able to drive them off by attacking from the rear and kill about a third of the Gunner Force that was currently attacking. However, a dozen Gunners were able to retreat.
Hollis moved his force into the town, where Sturges registered his unease about how few of them there were. Preston Garvey worked with Sturges to reinforce defenses and build walkways between buildings. While the Gunners attack the walls daily, the town had enough ammunition and defenses to hold out in the short term. Hollis requested more back-up from other units, none of whom arrived due to internal disputes. Veteran Minuteman Clint arrived at the Quincy gates, but while Hollis believed he was answering the call for reinforcements, Clint revealed he had decided to defect to the Gunners and was now the one leading the attack. Unsuccessful at convincing Col. Hollis and the mayor to stand down, Clint leaves to renew the assault.
Under the leadership of Clint, the Gunners attack Quincy from the overhanging highway under the cover of darkness, turning the town into a shooting gallery. The battle quickly devolved into a massacre for the Minutemen and inhabitants of Quincy, and was over in under an hour. All of the town's civilians and minutemen were killed except for Preston Garvey and nineteen others, who were able to escape to Jamaica Plain. Hollis was taken prisoner and killed by Clint.
The battle dealt a killing blow to the Commonwealth Minutemen as they were, with the movement too few in number and rife with distrust to hold together. Those that remained who did not arrive to defend Quincy faded away.
The Gunners dispatched a further detail to finish off Preston's group. They killed several more of them at Jamaica Plain reducing group numbers to a little over ten, but were unable to stop the rest of them from escaping. Preston's continued attempts to protect his group of survivors eventually took them to the ruins of Lexington. Feral ghouls whittled their number down, killing a settler, two other minutemen and Anthony, and little later Josh and Emma, before Jared's raider band, lead by Gristle, killed another Minuteman and forced the five survivors to take shelter in Concord's Museum of Freedom.
Clint was promoted to commander of Quincy by Captain Wes for his actions. Determined that the town would not be taken in the same way it had fallen to the Gunners, he fortified the highway, set up a radio tower and captured two sets of power armor in order to defend it.
If the Sole Survivor visits Quincy, they will see the armor being worn by Clint himself, and the other set by Tessa, who wields Tessa's Fist (a modified power armor piece). If the Survivor so chooses, the town can be attacked and Clint, Baker and Tessa can all be killed to avenge the Minutemen.
Despite what Preston believes, it seems that not many people know of the Quincy Massacre. None of the Diamond City guards comment about the massacre, and Piper successfully convinces Danny Sullivan to open the gate by convincing him that the Sole Survivor is a trader from Quincy.